The fact that Larian already said the Origin characters you do not take with you after Act 1 will not be playable after you leave for Moonrise (similar to DOS2), also confirms this. Once you depart for Moonrise, you will not return to this area and you will not be able to swap out to unused Origin characters.
Has that been absolutely confirmed? What an awful decision if so.
Why take on one of the most lauded RPG series of all time if only to dispense with so much of what made those previous games so memorable.